ORGANIC LETTERS
Asymmetric Total Synthesis of Danicalipin A and Evaluation of Biological Activity
2011 Vol. 13, No. 5 904–907
Taiki Umezawa,* Masayuki Shibata, Kensuke Kaneko, Tatsufumi Okino, and Fuyuhiko Matsuda* Division of Environmental Materials Science, Graduate School of Environmental Science, Hokkaido University, Sapporo 060-0810, Japan
[email protected];
[email protected] Received November 29, 2010
ABSTRACT
Asymmetric total synthesis of danicalipin A was achieved. The synthesis was characterized by diastereoselective introduction of chlorine atoms. Biological activities with synthetic danicalipin A, its enantiomer, and racemate were also evaluated toward brine shrimp. Both enantiomers of danicalipin A showed almost the same activity.
Danicalipin A (1), a major component of a group of chlorosulfolipids (CSLs) obtained from Ochromonas danica, was first detected more than four decades ago along with its congeners, representing a new class of lipids with one to six chlorine atoms (Figure 1).1 Although the planar structure of 1 was reported in 1973,1a the absolute and relative stereochemistries of 1 were not determined until 2009. In that year, Vanderwal reported the determination of the relative stereochemistries of 1 based on a total synthesis of a racemic mixture and of the absolute configuration using naturally derived material.2 Meanwhile, Okino described an absolute structural determination of 1 from a natural sample obtained from cultured O. danica using a combination of J-based configuration analysis (1) (a) Haines, T. H. Annu. Rev. Microbiol. 1973, 27, 403–411. (b) Mayers, G. L.; Haines, T. H. Biochemistry 1967, 6, 1665–1671. (c) Haines, T. H.; Pousada, M.; Stern, B.; Mayers, G. L. Biochem. J. 1969, 113, 565–566. (d) Elovson, J.; Vagelos, P. R. Proc. Natl. Acad. Sci. U.S.A. 1969, 62, 957–963. (e) Elovson, J.; Vagelos, P. R. Biochemistry 1970, 9, 3110–3126. (f) Elovson, J. Biochemistry 1974, 13, 3483– 3487. (2) Bedke, D. K.; Shibuya, G. M.; Pereira, A.; Gerwick, W. H.; Haines, T. H.; Vanderwal, C. D. J. Am. Chem. Soc. 2009, 131, 7570– 7572. (3) Kawahara, T.; Kumaki, Y.; Kamada, T.; Ishii, T.; Okino, T. J. Org. Chem. 2009, 74, 6016–6024. 10.1021/ol102882a r 2011 American Chemical Society Published on Web 02/03/2011
Figure 1. Chlorosulfolipids (CSLs).
(JBCA) and the modified Mosher’s method.3 These two reports reached the same conclusion: (11S, 13R, 14S, 15S, 16R), as shown in Figure 1. It has been reported that 1 shows a wide range of biological activities, such as toxicity
to fish4 and invertebrates,5 inhibition of bacterial growth, and lysis of mammalian erythrocytes.6 However, the biological mechanism of 1 has not been revealed due to the difficulty of obtaining molecular probes derived from natural sources. Therefore, elucidation of the mechanism using an artificially synthesized molecular probe is highly desirable. Due to the unprecedented structure and interesting biological activity of 1, along with other CSLs (2-5),7 these compounds have been the subject of much attention from synthetic chemists.8 In 2009, Carreira accomplished an elegant first total synthesis of chlorosulfolipid 2.9 After this, total syntheses of 1-3 have been achieved by two groups: racemic 1 and optically pure 3 by Vanderwal2,10 and optically active 2 by Yoshimitsu and Tanaka.11 In this report, we describe an asymmetric total synthesis of danicalipin A (1) and explore the biological activity of 1 and its enantiomer toward brine shrimp. For the total synthesis of 1, although a ring-opening reaction of a cis-epoxide by a chloride would be expected to be effective for the construction of syn-chlorohydrin at C13 and C14, Carreira first reported that epoxide ring-opening using substrates containing intramolecular chlorides resulted in an undesired product of diastereomers due to the generation of intramolecular chloronium ions (Figure 2, eq 1).9a Vanderwal also observed the similar phenomenon.2 To avoid this side reaction, an epoxide ringopening reaction using a substrate without chloride may be employed at an early stage of the total synthesis. Meanwhile, to synthesize the dichloride at C15 and C16 with anti configuration, anti-addition of a molecular chlorine equivalent to an E-olefin was considered. However, because a previous study by Yoshimitsu and Tanaka indicated that the addition produced a syn-adduct along with the anti-adducts,11a improvement of the selectivity of the anti-addition reaction was concluded to be important (4) (a) Reich, K.; Spiegelstein, M. Isr. J. Zool. 1964, 13, 141. (b) Leeper, D. A.; Porter, K. G. Arch. Hydrobiol. 1995, 134, 207–222. (5) (a) Boxhorn, J. E.; Holen, D. A.; Boraas, M. E. Hydrobiologia 1998, 387/388, 283–287. (b) Boenigk, J.; Stadler, P. J. Plankton Res. 2004, 26, 1507–1514. (6) (a) Hansen, J. A. Physiol. Plant. 1973, 29, 234–238. (b) Halevy, S.; Saliternik, R.; Avivi, L. Int. J. Biochem. 1971, 2, 185–192. (c) Magazanik, A.; Halevy, S. Experientia 1973, 15, 310–311. (7) (a) Ciminiello, P.; Fattorusso, E.; Forino, M. J. Org. Chem. 2001, 66, 578–582. (b) Chen, J. L.; Proteau, P. J.; Roberts, M. A.; Gerwick, W. H.; Slate, D. L.; Lee, R. H. J. Nat. Prod. 1994, 57, 524–527. (c) Ciminiello, P.; Dell’Aversano, C.; Fattorusso, E.; Forino, M.; Magno, S.; Di Meglio, P.; Ianaro, A.; Poletti, R. J. Am. Chem. Soc. 2002, 124, 13114–13120. (d) Ciminiello, P.; Dell’Aversano, C.; Fattorusso, E.; Forino, M.; Magno, S.; Di Meglio, P.; Ianaro, A.; Poletti, R. Tetrahedron 2004, 60, 7093–7098. (e) Pereira, A. R.; Byrum, T.; Shibuya, G. M.; Vanderwal, C. D.; Gerwick, W. H. J. Nat. Prod. 2010, 73, 279– 283. (8) Denton, R. M.; Tang, X.; Przeslak, A. Org. Lett. 2010, 12, 4678– 4681. (9) (a) Nilewski, C.; Geisser, R. W.; Carreira, E. M. Nature 2009, 457, 573–576. (b) Nilewski, C.; Geisser, R. W.; Ebert, M.-O.; Carreira, E. M. J. Am. Chem. Soc. 2009, 131, 15866–15876. (10) (a) Bedke, D. K.; Shibuya, G. M.; Pereira, A.; Gerwick, W. H.; Vanderwal, C. D. J. Am. Chem. Soc. 2010, 132, 2542–2543. (b) Kanady, J. S.; Nguyen, J. D.; Ziller, J. W.; Vanderwal, C. D. J. Org. Chem. 2009, 74, 2175–2178. (c) Shibuya, G. M.; Kanady, J. S.; Vanderwal, C. D. J. Am. Chem. Soc. 2008, 130, 12514–12518. (d) Bedke, D. K.; Vanderwal, C. D. Nat. Prod. Rep. 2011, 28, 15–25. (11) (a) Yoshimitsu, T.; Fukumoto, N.; Nakatani, R.; Kojima, N.; Tanaka, T. J. Org. Chem. 2010, 75, 5425–5437. (b) Yoshimitsu, T.; Fukumoto, N.; Tanaka, T. J. Org. Chem. 2009, 74, 696–702. Org. Lett., Vol. 13, No. 5, 2011
Figure 2. Previous synthetic problems to overcome.
Scheme 1. Retrosynthetic Analysis of Danicalipin A
(Figure 2, eq 2). With these considerations in mind, the following retrosynthetic analysis of 1 was planned (Scheme 1). As mentioned above, 1 was expected to be obtained from E-olefin 6 by anti-addition of the molecular chlorine equivalent, which would be derived from aldehyde 7 and phosphonium salt 8 by Wittig olefination. We envisioned that aldehyde 7 would be accessed by R-chlorination with aldehyde 9, as reported by Jørgensen,12 which would be synthesized from the known cis-epoxide 10. 905
Scheme 2. Synthesis of Aldehyde 7
Scheme 3. Synthesis of Phosphonium Salt 8
The synthesis of 1 commenced with the known chiral epoxide 10,13 derived via Sharpless-Katsuki asymmetric epoxidation, which was transformed into R,β-unsaturated ester 12 in a one-pot operation (Scheme 2).14 At this stage, the enantiomeric excess of 12 was enhanced from 85% ee to >99% ee by recrystallization. Regioselective ring-opening of the epoxide by chloride was achieved by treatment with SOCl2 to give chlorohydrin 13 as a sole product in excellent yield.15 Reduction of the R,β-unsaturated ester with LiBH4 afforded a saturated primary alcohol, the alcohol groups of which were protected as TES ethers. TES ether 14 was then converted to aldehyde 9, the precursor for R-chlorination, by Swern oxidation. After extensive investigations of catalysts, additives, and temperatures for the diastereoselective R-chlorination reaction, the combination of (R,R)-2,5-diphenylpyrrolidine (15)12 and bromoacetic acid in 1,2-dichloroethane at 0 °C was found to achieve the best selectivity (diastereoselectivity >20:1; ratio of mono- to dichlorination >20:1), furnishing a labile R-chloroaldehyde which was isolated as R,β-unsaturated ester 16 by adding a Wittig reagent in a one-pot operation. The R,β-unsaturated ester
16 was converted to 7 by LiBH 4 reduction of the R,β-unsaturated ester to a saturated alcohol, followed by TEMPO oxidation. With 7 in hand, we next turned our attention to the synthesis of another coupling partner, the phosphonium salt 8. Clean conversion of the known aldehyde 1116 to R,Rdichloroaldehyde 17 was accomplished by treatment with NCS in the presence of a catalytic amount of pyrrolidine at 60 °C without the formation of byproduct such as R-monochloroaldehyde or aldol products (Scheme 3). DIBAL reduction and subsequent protection of the resultant alcohol gave TBS ether 18, which was converted into 8 in two steps. Wittig reaction between 7 and 8 furnished olefin in 83% yield (Scheme 4). Hydrogenation and deprotection of the PMB group using Pearlman’s catalyst gave alcohol 19, which was transformed to R,β-unsaturated ester 20 with E geometry as a major product by treatment with a Wittig reagent17 derived from (n-Bu)3P after generation of aldehyde. To convert the ester into an n-hexyl group, 20 was converted into acetate 21 and allylic substitution18 of 21 with n-C5H11MgBr and Li2CuCl4 afforded olefin 22. Because dichlorination with 22 resulted in a complex mixture, the silyl ethers were removed. After numerous investigations of the anti-selective dichlorination reaction under Mark o’s conditions,19 we found that an anti-addition reaction of a molecular chlorine equivalent took place with high anti selectivity when the reaction was carried out in octane at 90 °C. The desired hexachloride 23 was obtained in 39% yield along with another anti-adduct (28%) as a minor diastereomer after purification by HPLC. All spectroscopic data of 23 were completely identical with those of an intermediate synthesized by Vanderwal. According to Vanderwal’s synthesis, two sulfate groups were introduced into two hydroxyl groups in 23 by treatment with ClSO3H, achieving the asymmetric total synthesis of danicalipin A (1). The [R]D value of synthetic 1 was
(12) Halland, N.; Braunton, A.; Bachmann, S.; Marigo, M.; Jørgensen, K. A. J. Am. Chem. Soc. 2004, 126, 4790–4791. (13) Trost, B. M.; Wrobleski, S. T.; Chisholm, J. D.; Harrington, P. E.; Jung, M. J. Am. Chem. Soc. 2005, 127, 13589–13597. (14) Vatele, J.-M. Tetrahedron Lett. 2006, 47, 715–718. (15) Barluenga, S.; Moulin, E.; Lopez, P.; Winssinger, N. Chem.; Eur. J. 2005, 11, 4935–4952.
(16) Ballini, R.; Bosica, G.; Rafaian, G. Helv. Chim. Acta 1995, 78, 879–882. (17) Harcken, C.; Martin, S. F. Org. Lett. 2001, 3, 3591–3593. (18) B€ackvall, J.-E.; Sellen, M.; Grant, B. J. Am. Chem. Soc. 1990, 112, 6615–6621. (19) Mark o, I. E.; Richardson, P. R.; Bailey, M.; Maguire, A. R.; Coughlan, N. Tetrahedron Lett. 1997, 38, 2339–2342.
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Scheme 4. Total Synthesis of Danicalipin A
Table 1. Toxicity of Synthetic 1, ent-1, and rac-1 in Brine Shrimp entry
compound
LC50 (μg/mL)a
1 2 3 4
Natural 1b Synthetic 1 Synthetic ent-1 Synthetic rac-1c
2.2 2.1 2.4 2.4
c
a LC50: median lethal concentration against brine shrimp. b See ref 3. Prepared by mixing 1 and ent-1 in a 1:1 ratio.
configuration of danicalipin A has no effect on its toxicity in brine shrimp. In summary, enantioselective total syntheses of both enantiomers of danicalipin A were achieved in 17 steps (the longest linear sequence) from the known epoxide 10. The synthesis featured regioselective construction of syn-chlorohydrin via epoxide, diastereoselective R-chlorination of aldehyde, and dichlorination of E-olefin. LC50 evaluation of 1 and ent-1 revealed that the stereochemistry of 1 did not affect its activity. Syntheses of analogues of 1 using a modified Grignard reagent in allylic substitution reactions and biological testing of the products are currently underway. þ31.5 (c = 0.25, MeOH), which corresponded to the reported [R]D value [þ38 (c = 0.78)].2 Next, we investigated the toxicities of 1, ent-1;which was synthesized via the same scheme from ent-10;and rac-1. Table 1 shows the LC50 values (50% lethal concentration) for brine shrimp. The biological activity of synthetic 1 (LC50 = 2.1 μg/mL) was identical to that of a natural sample. The enantiomer ent-1 and the rac-1 showed almost the same activity (LC50 = 2.4 μg/mL) as natural 1. These results clearly indicate that the absolute
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Acknowledgment. This work was financially supported by a research grant for young scientists from the Global COE program “Establishment of Center for Integrated Field Environmental Science”. Supporting Information Available. Experimantal procedure and copies of 1H NMR and 13C NMR spectra of new compounds. This material is available free of charge via the Internet at http://pubs.acs.org.
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